Targeting anionic phospholipids on tumor blood vessels and tumor cells

Integrin Targeting Enhances the Antimelanoma Effect of Annexin V in Mice

Malignant melanoma, an increasingly common form of skin cancer, is a major threat to public health, especially when the disease progresses past skin lesions to the stage of advanced metastasis. Targeted drug development is an effective strategy for the treatment of malignant melanoma. In this work, a new antimelanoma tumor peptide, the lebestatin-annexin V (designated LbtA5) fusion protein, was developed and synthesized by recombinant DNA techniques. As a control, annexin V (designated ANV) was also synthesized by the same method. The fusion protein combines annexin V, which specifically recognizes and binds phosphatidylserine, with the disintegrin lebestatin (lbt), a polypeptide that specifically recognizes and binds integrin α1β1. LbtA5 was successfully prepared with good stability and high purity while retaining the dual biological activity of ANV and lbt. MTT assays demonstrated that both ANV and LbtA5 could reduce the viability of melanoma B16F10 cells, but the activity of the fusion protein LbtA5 was superior to that of ANV. The tumor volume growth was slowed in a mouse xenograft model treated with ANV and LbtA5, and the inhibitory effect of high concentrations of LbtA5 was significantly better than that of the same dose of ANV and was comparable to that of DTIC, a drug used clinically for melanoma treatment. The hematoxylin and eosin (H&E) staining test showed that ANV and LbtA5 had antitumor effects, but LbtA5 showed a stronger ability to induce melanoma necrosis in mice. Immunohistochemical experiments further showed that ANV and LbtA5 may inhibit tumor growth by inhibiting angiogenesis in tumor tissue. Fluorescence labeling experiments showed that the fusion of ANV with lbt enhanced the targeting of LbtA5 to mouse melanoma tumor tissue, and the amount of target protein in tumor tissue was significantly increased. In conclusion, effective coupling of the integrin α1β1-specific recognition molecule lbt confers stronger biological antimelanoma effects of ANV, which may be achieved by the dual effects of effective inhibition of B16F10 melanoma cell viability and inhibition of tumor tissue angiogenesis. The present study describes a new potential strategy for the application of the promising recombinant fusion protein LbtA5 in the treatment of various cancers, including malignant melanoma.

Targeting Xkr8 via nanoparticle-mediated in situ co-delivery of siRNA and chemotherapy drugs for cancer immunochemotherapy

Activation of scramblases is one of the mechanisms that regulates the exposure of phosphatidylserine to the cell surface, a process that plays an important role in tumour immunosuppression. Here we show that chemotherapeutic agents induce overexpression of Xkr8, a scramblase activated during apoptosis, at the transcriptional level in cancer cells, both in vitro and in vivo. Based on this finding, we developed a nanocarrier for co-delivery of Xkr8 short interfering RNA and the FuOXP prodrug to tumours. Intravenous injection of our nanocarrier led to significant inhibition of tumour growth in colon and pancreatic cancer models along with increased antitumour immune response. Targeting Xkr8 in combination with chemotherapy may represent a novel strategy for the treatment of various types of cancers.

Transferrin-conjugated liposomes loaded with carnosic acid inhibit liver cancer growth by inducing mitochondria-mediated apoptosis

Our previous studies have proven that carnosic acid (CA) induces apoptosis of liver cancer cells. However, the poor chemical properties of CA limit its in vivo anti-cancer effects. In this study, CA was loaded into liposomes (LP-CA), and LP-CA was further conjugated with transferrin (Tf-LP-CA) to overcome the shortcomings of poor solubility and absorption at the lesion site. In HepG2 and SMMC-7721 cells, compared with CA and LP-CA, more Tf-LP-CA was absorbed by liver cancer cells, which induced higher levels of apoptosis and reduced the mitochondrial membrane potential more effectively. In HepG2- and SMMC-7721-xenotransplanted mice, Tf-LP-CA inhibited tumor growth with no cytotoxicity to the liver, spleen, or kidney. Furthermore, compared with CA and LP-CA, Tf-LP-CA targeted the tumor site more effectively, enhanced the expressions of cleaved poly(ADP-ribose) polymerase, and Caspase-3 and -9, and regulated the expression levels of B-cell lymphoma 2 (Bcl2) family members in the tumor tissues. Tf-LP-CA was taken up by tumor cells and targeted at tumor tissues, ensuring the precise delivery of CA, which further promoted mitochondria-mediated intrinsic apoptosis in the liver cancer cells. These results provide evidence for the clinical application of the Tf-LP-based CA drug delivery system for liver cancer.

A brief review of the essential role of nanovehicles for improving the therapeutic efficacy of pharmacological agents against tumours

Cancer is the leading cause of death globally. There are several differences between cancer cells and normal cells. From all the therapies, chemotherapy is the most prominent therapy to treat cancer. However, the conventional drug delivery that is used to deliver poorly aqueous soluble chemotherapeutic agents has several obstacles such as whole-body distribution, rapid excretion, degradation before reaching the infected site, side effects, etc. Nanoformulation of these aqueous insoluble agents is the emerging delivery system for targeted and increasing solubility. Among all the three methods (physical, chemical and biological) chemical and biological methods are mostly used for the synthesis of nanovehicles (NVs) of different sizes, shapes and dimensions. A passive targeting delivery system in which NVs supports the pharmacological agents (drugs/genes) is a good way for resolving the obstacles with a conventional delivery system. It enhances the therapeutic efficacy of pharmacological agents (drugs/genes). These NVs have several specific characters like small size, large surface area to volume ratio, surface functionalization, etc. However, this delivery is not able to deliver site-specific delivery of drugs. An active targeting delivery system in which pharmacological agents are loaded on NVs to attack directly on cancer cells and tissues is a superior way for delivering the pharmacological agents compared to a passive targeting delivery system. Various targeting ligands have been investigated and applied for targeting the delivery of drugs such as sugar, vitamin, antibodies, protein, peptides, etc. These targeted ligand supports to guide the NVs accumulated directly on the cancer cells with a higher level of cellular internalization compared to passive targeting and conventional delivery system.

Linker Optimization and Therapeutic Evaluation of Phosphatidylserine-Targeting Zinc Dipicolylamine-based Drug Conjugates

We report that compound 13, a novel phosphatidylserine-targeting zinc(II) dipicolylamine drug conjugate, readily triggers a positive feedback therapeutic loop through the in situ generation of phosphatidylserine in the tumor microenvironment. Linker modifications, pharmacokinetics profiling, in vivo antitumor studies, and micro-Western array of treated-tumor tissues were employed to show that this class of conjugates induced regeneration of apoptotic signals, which facilitated subsequent recruitment of the circulating conjugates through the zinc(II) dipicolylamine-phosphatidylserine association and resulted in compounding antitumor efficacy. Compared to the marketed compound 17, compound 13 not only induced regressions in colorectal and pancreatic tumor models, it also exhibited at least 5-fold enhancement in antitumor efficacy with only 40% of the drug employed during treatment, culminating in a >12.5-fold increase in therapeutic potential. Our study discloses a chemically distinct apoptosis-targeting theranostic, with built-in complementary functional moieties between the targeting module and the drug mechanism to expand the arsenal of antitumor therapy.

Anti-cancer activity of Annexin V in murine melanoma model by suppressing tumor angiogenesis

Annexin V, a protein with high affinity to phosphatidylserine (PS) in a calcium dependent manner, has been widely used to probe apoptosis. Annexin V in inhibiting engulfment of apoptotic cells by macrophages had been reported to increase the immunogenicity of tumor cells undergoing apoptosis. However, far less is known about its multiple properties, especially in cancer therapies. Here we found that Annexin V had a good anti-tumor activity in murine melanomaxenograft model. Treatment with Annexin V showed significant reduction in tumor size and remarkable tumor necrosis areas. The serum level of VEGF was downregualted by Annexin V both in normal mice and mice bearing tumor, suggesting that its new role on impeding tumor angiogenesis. In Silico analysis using Oncomine database, we also found the negative correlation of AnnexinV and VEGF both in skin and melanoma. The decreased Annexin V expression shows linearity relation with the elevated VEGF expression. These data provided a possibility that Annexin V can be used as a novel angiogenesis inhibitor in tumor therapy.

Protective Effect of Anti-Phosphatidylserine Antibody in a Guinea Pig Model of Advanced Hemorrhagic Arenavirus Infection

Objective

Host derived markers on virally infected cells or virions may provide targets for the generation of antiviral agents. Recently, we identified phosphatidylserine (PS) as a host marker of virions and virally-infected cells.

Methods and Materials

Under normal physiological conditions, PS is maintained on the inner leaflet of the plasma membrane facing the cytosol. Following viral infection, activation or pre-apoptotic changes cause PS to become externalized. We have previously shown that bavituximab, a chimeric human-mouse antibody that binds PS complexed with β2-glycoprotein I (β2GP1), protected rodents against lethal Pichinde virus and cytomegalovirus infections.

Results

Here, we determined the antiviral activity of a fully human monoclonal antibody, PGN632, that directly binds to PS. Treatment with PGN632 protected 20% of guinea pigs with advanced infections of the hemorrhagic arenavirus, Pichinde, from death. Combining PGN632 with ribavirin improved the antiviral activity of both agents, such that the combination rescued 50% of animals from death.

Conclusion

The major mechanisms of action of PGN632 appear to be opsonization of virus and antibody-dependent cellular cytotoxicity of virally-infected cells. PS-targeting agents may have utility in the treatment of viral diseases.

A Phase I Clinical Trial of the Phosphatidylserine-targeting Antibody Bavituximab in Combination With Radiation Therapy and Capecitabine in the Preoperative Treatment of Rectal Adenocarcinoma

OBJECTIVES

There is interest in improving the tumoricidal effects of preoperative radiotherapy for rectal carcinoma by studying new radiosensitizers. The safety and toxicity profile of these combination regimens needs rigorous clinical evaluation. The primary objective of this study was to evaluate the toxicity of combining bavituximab, an antibody that targets exposed phosphatidylserine, with capecitabine and radiation therapy.

MATERIALS AND METHODS

Patients with stage II or III rectal adenocarcinoma were enrolled on a phase I study combining radiation therapy, capecitabine, and bavituximab. A standard 3+3 trial designed was used.

RESULTS

In general, bavituximab was safe and well tolerated in combination with radiation therapy and capecitabine in the treatment of rectal adenocarcinoma. One patient at the highest dose level experienced a grade III infusion reaction related to the bavituximab. One tumor demonstrated a complete pathologic response to the combination treatment.

CONCLUSIONS

Bavituximab is safe in combination with capecitabine and radiation therapy at the doses selected for the study. Further clinical investigation would be necessary to better define the efficacy of this combination.

Targeting Tumor Associated Phosphatidylserine with New Zinc Dipicolylamine-Based Drug Conjugates

A series of zinc(II) dipicolylamine (ZnDPA)-based drug conjugates have been synthesized to probe the potential of phosphatidylserine (PS) as a new antigen for small molecule drug conjugate (SMDC) development. Using in vitro cytotoxicity and plasma stability studies, PS-binding assay, in vivo pharmacokinetic studies, and maximum tolerated dose profiles, we provided a roadmap and the key parameters required for the development of the ZnDPA based drug conjugate. In particular, conjugate 24 induced tumor regression in the COLO 205 xenograft model and exhibited a more potent antitumor effect with a 70% reduction of cytotoxic payload compared to that of the marketed irinotecan when dosed at the same regimen. In addition to the validation of PS as an effective pharmacodelivery target for SMDC, our work also provided the foundation that, if applicable, a variety of therapeutic agents could be conjugated in the same manner to treat other PS-associated diseases.

Therapeutics Against Filovirus Infection

Therapies for filovirus infections are urgently needed. The paradoxical issue facing therapies is the need for rigorous safety and efficacy testing, adhering to the principle tenant of medicine to do no harm, while responding to the extreme for a treatment option during an outbreak. Supportive care remains a primary goal for infected patients. Years of research into filoviruses has provided possible medical interventions ranging from direct antivirals, host-factor supportive approaches, and passive immunity. As more basic research is directed toward understanding these pathogens and their impact on the host, effective approaches to treat patients during infection will be identified. The ability to manage outbreaks with medical interventions beyond supportive care will require clinical trial design that will balance the benefits of the patient and scientific community.

Variation in human cancer cell external phosphatidylserine is regulated by flippase activity and intracellular calcium

Viable cancer cells expose elevated levels of phosphatidylserine (PS) on the exoplasmic face of the plasma membrane. However, the mechanisms leading to elevated PS exposure in viable cancer cells have not been defined. We previously showed that externalized PS may be used to monitor, target and kill tumor cells. In addition, PS on tumor cells is recognized by macrophages and has implications in antitumor immunity. Therefore, it is important to understand the molecular details of PS exposure on cancer cells in order to improve therapeutic targeting. Here we explored the mechanisms regulating the surface PS exposure in human cancer cells and found that differential flippase activity and intracellular calcium are the major regulators of surface PS exposure in viable human cancer cells. In general, cancer cell lines with high surface PS exhibited low flippase activity and high intracellular calcium, whereas cancer cells with low surface PS exhibited high flippase activity and low intracellular calcium. High surface PS cancer cells also had higher total cellular PS than low surface PS cells. Together, our results indicate that the amount of external PS in cancer cells is regulated by calcium dependent flippase activity and may also be influenced by total cellular PS.

Stereotactic body radiotherapy for the pancreas: a critical review for the medical oncologist

With recent advances in imaging modalities and radiation therapy, stereotactic body radiotherapy (SBRT) has allowed for the delivery of high doses of radiation with accuracy and precision. As such, SBRT has generated favorable results in the treatment of several cancers. Although the role of radiation has been controversial for the treatment of pancreatic ductal adenocarcinoma (PDAC) due to rather lackluster results in clinical trials, SBRT may offer improved outcomes, enhance the quality of life, and aid in palliative care settings for PDAC patients. This review delineates the role of SBRT in the treatment of PDAC, presents the defining principles of radiation biology and the radiation oncology work flow, and discusses the prospects of new treatment regimens involving tumor immunology and radiation therapy.

Antibody-Mediated Blockade of Phosphatidylserine Enhances the Antitumor Effect of Sorafenib in Hepatocellular Carcinomas Xenografts

BACKGROUND

Currently, the only FDA-approved systemic therapy for hepatocellular carcinoma (HCC) is the multi-receptor tyrosine kinase inhibitor, sorafenib, which provides only modest clinical benefit. We recently showed that treatment with a phosphatidylserine (PS) targeting agent suppresses tumor growth by targeting tumor vasculature and reactivating antitumor immunity.

METHODS

We tested the hypothesis that sorafenib increases PS exposure on tumor vasculature, thereby enhancing the antitumor efficacy of PS targeting. We evaluated the efficacy of combining a PS targeting agent (2aG4) with sorafenib in murine xenograft models of human HCC.

RESULTS

Our results demonstrate that combination of 2aG4 and sorafenib had a superior therapeutic effect over single agent therapy. Mechanistic studies showed that sorafenib significantly increased PS exposure on tumor vasculature; the percentage of PS-positive vessels increased from 19 to 52, 23 to 68, and 30 to 55 % in PLC/PRF/5, C3A, and Huh7 tumors, respectively. Combination therapy significantly decreased tumor microvessel density and the level of M2 macrophages, while increasing the apoptotic index of tumor endothelial cells and the frequency of M1 macrophages. Furthermore, we report the findings of a Phase I clinical study of bavituximab, a chimeric version of 2aG4, combined with sorafenib in HCC patients. The Phase I results demonstrate the appropriate dose of bavituximab to be given with sorafenib in future clinical trials.

CONCLUSIONS

Overall, these results strongly support the combination of bavituximab with sorafenib as a promising systemic therapeutic strategy for the treatment for advanced HCC patients.

Low-Energy Ultrasound Treatment Improves Regional Tumor Vessel Infarction by Retargeted Tissue Factor

OBJECTIVES

To enhance the regional antitumor activity of the vascular-targeting agent truncated tissue factor (tTF)-NGR by combining the therapy with low-energy ultrasound (US) treatment.

METHODS

For the in vitro US exposure of human umbilical vein endothelial cells (HUVECs), cells were put in the focus of a US transducer. For analysis of the US-induced phosphatidylserine (PS) surface concentration on HUVECs, flow cytometry was used. To demonstrate the differences in the procoagulatory efficacy of TF-derivative tTF-NGR on binding to HUVECs with a low versus high surface concentration of PS, we performed factor X activation assays. For low-energy US pretreatment, HT1080 fibrosarcoma xenotransplant-bearing nude mice were treated by tumor-regional US-mediated stimulation (ie, destruction) of microbubbles. The therapy cohorts received the tumor vessel-infarcting tTF-NGR protein with or without US pretreatment (5 minutes after US stimulation via intraperitoneal injection on 3 consecutive days).

RESULTS

Combination therapy experiments with xenotransplant-bearing nude mice significantly increased the antitumor activity of tTF-NGR by regional low-energy US destruction of vascular microbubbles in tumor vessels shortly before application of tTF-NGR (P < .05). Mechanistic studies proved the upregulation of anionic PS on the outer leaflet of the lipid bilayer of endothelial cell membranes by low-energy US and a consecutive higher potential of these preapoptotic endothelial cells to activate coagulation via tTF-NGR and coagulation factor X as being a basis for this synergistic activity.

CONCLUSIONS

Combining retargeted tTF to tumor vessels with proapoptotic stimuli for the tumor vascular endothelium increases the antitumor effects of tumor vascular infarction. Ultrasound treatment may thus be useful in this respect for regional tumor therapy.

Phosphatidylserine-selective targeting and anticancer effects of SapC-DOPS nanovesicles on brain tumors

Brain tumors, either primary (e.g., glioblastoma multiforme) or secondary (metastatic), remain among the most intractable and fatal of all cancers. We have shown that nanovesicles consisting of Saposin C (SapC) and dioleylphosphatidylserine (DOPS) are able to effectively target and kill cancer cells both in vitro and in vivo. These actions are a consequence of the affinity of SapC-DOPS for phosphatidylserine, an acidic phospholipid abundantly present in the outer membrane of a variety of tumor cells and tumor-associated vasculature. In this study, we first characterize SapC-DOPS bioavailability and antitumor effects on human glioblastoma xenografts, and confirm SapC-DOPS specificity towards phosphatidylserine by showing that glioblastoma targeting is abrogated after in vivo exposure to lactadherin, which binds phosphatidylserine with high affinity. Second, we demonstrate that SapC-DOPS selectively targets brain metastases-forming cancer cells both in vitro, in co-cultures with human astrocytes, and in vivo, in mouse models of brain metastases derived from human breast or lung cancer cells. Third, we demonstrate that SapC-DOPS nanovesicles have cytotoxic activity against metastatic breast cancer cells in vitro, and prolong the survival of mice harboring brain metastases. Taken together, these results support the potential of SapC-DOPS for the diagnosis and therapy of primary and metastatic brain tumors.

Effective binding of a phosphatidylserine-targeting antibody to Ebola virus infected cells and purified virions

Ebola virus is responsible for causing severe hemorrhagic fevers, with case fatality rates of up to 90%. Currently, no antiviral or vaccine is licensed against Ebola virus. A phosphatidylserine-targeting antibody (PGN401, bavituximab) has previously been shown to have broad-spectrum antiviral activity. Here, we demonstrate that PGN401 specifically binds to Ebola virus and recognizes infected cells. Our study provides the first evidence of phosphatidylserine-targeting antibody reactivity against Ebola virus.

Systemic delivery of SapC-DOPS has antiangiogenic and antitumor effects against glioblastoma

Saposin C-dioleoylphosphatidylserine (SapC-DOPS) nanovesicles are a nanotherapeutic which effectively target and destroy cancer cells. Here, we explore the systemic use of SapC-DOPS in several models of brain cancer, including glioblastoma multiforme (GBM), and the molecular mechanism behind its tumor-selective targeting specificity. Using two validated spontaneous brain tumor models, we demonstrate the ability of SapC-DOPS to selectively and effectively cross the blood-brain tumor barrier (BBTB) to target brain tumors in vivo and reveal the targeting to be contingent on the exposure of the anionic phospholipid phosphatidylserine (PtdSer). Increased cell surface expression of PtdSer levels was found to correlate with SapC-DOPS-induced killing efficacy, and tumor targeting in vivo was inhibited by blocking PtdSer exposed on cells. Apart from cancer cell killing, SapC-DOPS also exerted a strong antiangiogenic activity in vitro and in vivo. Interestingly, unlike traditional chemotherapy, hypoxic cells were sensitized to SapC-DOPS-mediated killing. This study emphasizes the importance of PtdSer exposure for SapC-DOPS targeting and supports the further development of SapC-DOPS as a novel antitumor and antiangiogenic agent for brain tumors.

Catheterized guinea pigs infected with Ebola Zaire virus allows safer sequential sampling to determine the pharmacokinetic profile of a phosphatidylserine-targeting monoclonal antibody

Sequential sampling from animals challenged with highly pathogenic organisms, such as haemorrhagic fever viruses, is required for many pharmaceutical studies. Using the guinea pig model of Ebola virus infection, a catheterized system was used which had the benefits of allowing repeated sampling of the same cohort of animals, and also a reduction in the use of sharps at high biological containment. Levels of a PS-targeting antibody (Bavituximab) were measured in Ebola-infected animals and uninfected controls. Data showed that the pharmacokinetics were similar in both groups, therefore Ebola virus infection did not have an observable effect on the half-life of the antibody.

Recent advances in the application of antibodies as therapeutics

The application of antibodies as therapeutic agents in the treatment of cancer now represents a significant proportion of the oncology drug arena. Despite this success, the ability to engineer and exploit antibodies in many different formats is ensuring that new avenues for their therapeutic application are constantly being examined. This review examines a selection of novel antibody-based therapeutic strategies that are currently in late preclinical and clinical evaluation.

High resolution tumor targeting in living mice by means of multispectral optoacoustic tomography

Background

Tumor targeting is of high clinical and biological relevance, and major efforts have been made to develop molecular imaging technologies for visualization of the disease markers in tissue. Of particular interest is apoptosis which has a profound role within tumor development and has significant effect on cancer malignancy.

Methods

Herein, we report on targeting of phosphatidylserine-exposing cells within live tumor allograft models using a synthetic near infrared zinc(II)-dipicolylamine probe. Visualization of the probe biodistribution is performed with whole body multispectral optoacoustic tomography (MSOT) system and subsequently compared to results attained by planar and tomographic fluorescence imaging systems.

Results

Compared to whole body optical visualization methods, MSOT attains remarkably better imaging capacity by delivering high-resolution scans of both disease morphology and molecular function in real time. Enhanced resolution of MSOT clearly showed that the probe mainly localizes in the vessels surrounding the tumor, suggesting that its tumor selectivity is gained by targeting the phosphatidylserine exposed on the surface of tumor vessels.

Conclusions

The current study demonstrates the high potential of MSOT to broadly impact the fields of tumor diagnostics and preclinical drug development.

Development of bavituximab, a vascular targeting agent with immune-modulating properties, for lung cancer treatment

Bavituximab is a chimeric monoclonal antibody directed against the membrane phospholipid phosphatidylserine. Phosphatidylserine exposure is increased on endothelial cells and apoptotic cancer cells in solid tumors, allowing tumor-specific targeting of bavituximab. Bavituximab binding results in tumor vessel occlusion and enhanced antitumor immunity. Preclinical investigations have demonstrated efficacy as monotherapy and in combination with other modalities against multiple cancer types. Phase I clinical trials of bavituximab monotherapy and in combination with chemotherapy in adults with refractory solid tumors have been completed. Phase II trials of bavituximab in combination with chemotherapy for the first- and second-line treatment of advanced non-small-cell lung cancer are currently ongoing. This article summarizes the preclinical development and clinical experience with bavituximab in non-small-cell lung cancer.

In vivo targeting of cell death using a synthetic fluorescent molecular probe

A synthetic, near-infrared, fluorescent probe, named PSS-794 was assessed for its ability to detect cell death in two animal models. The molecular probe contains a zinc(II)-dipicolylamine (Zn(2+)-DPA) affinity ligand that selectively targets exposed phosphatidylserine on the surface of dead and dying cells. The first animal model used rats that were treated with dexamethasone to induce thymic atrophy. Ex vivo fluorescence imaging and histological analysis of excised organs showed thymus uptake of PSS-794 was four times higher than a control fluorophore that lacked the Zn(2+)-DPA affinity ligand. In addition, the presence of PSS-794 produced a delayed and higher build up of dead and dying cells in the rat thymus. The second animal model employed focal beam radiation to induce cell death in tumor-bearing rats. Whole-body and ex vivo imaging showed that the amount of PSS-794 in a radiation-treated tumor was almost twice that in a non-treated tumor. The results indicate that PSS-794 may be useful for preclinical optical detection of tumor cell death due to therapy.

Alboserpin, a factor Xa inhibitor from the mosquito vector of yellow fever, binds heparin and membrane phospholipids and exhibits antithrombotic activity

The molecular mechanism of factor Xa (FXa) inhibition by Alboserpin, the major salivary gland anticoagulant from the mosquito and yellow fever vector Aedes albopictus, has been characterized. cDNA of Alboserpin predicts a 45-kDa protein that belongs to the serpin family of protease inhibitors. Recombinant Alboserpin displays stoichiometric, competitive, reversible and tight binding to FXa (picomolar range). Binding is highly specific and is not detectable for FX, catalytic site-blocked FXa, thrombin, and 12 other enzymes. Alboserpin displays high affinity binding to heparin (K(D) ~ 20 nM), but no change in FXa inhibition was observed in the presence of the cofactor, implying that bridging mechanisms did not take place. Notably, Alboserpin was also found to interact with phosphatidylcholine and phosphatidylethanolamine but not with phosphatidylserine. Further, annexin V (in the absence of Ca(2+)) or heparin outcompetes Alboserpin for binding to phospholipid vesicles, suggesting a common binding site. Consistent with its activity, Alboserpin blocks prothrombinase activity and increases both prothrombin time and activated partial thromboplastin time in vitro or ex vivo. Furthermore, Alboserpin prevents thrombus formation provoked by ferric chloride injury of the carotid artery and increases bleeding in a dose-dependent manner. Alboserpin emerges as an atypical serpin that targets FXa and displays unique phospholipid specificity. It conceivably uses heparin and phosphatidylcholine/phosphatidylethanolamine as anchors to increase protein localization and effective concentration at sites of injury, cell activation, or inflammation.